22-05-2013, 11:41 AM
Voltage Control with Wind Turbines
Introduction
Wind turbines have generally contributed a small embedded capacity within grid networks
and so node voltages are maintained within the allowed deviation from their nominal
values, by large central capacity, i.e. by the synchronous generators in power plants.
However, with the increase in capacity and number of wind parks, including large offshore
installations, the contribution from wind power to the overall demand may no longer be
considered small and it will start to replace the output of conventional generators. It will
therefore no longer be possible to control node voltages adequately by only using the
synchronous generators in large-scale conventional power stations and it is essential that
wind power also contributes to voltage control. This chapter reviews the essential and basic
principles of voltage control with wind turbines having either geared squirrel cage or
doubly fed induction generators or direct drive synchronous generators. First, a didactic
approach from the fundamentals of voltage control is given.
Chapter Outline
The chapter starts with an outline of the voltage control problem. It is explained that
voltage control is necessary due to the impedances of branches, i.e. lines and cables. Due to
these impedances, the voltage between both ends of a branch is not the same, but instead
there is a voltage difference between the two ends of the branch. The fact that the voltage is
not the same throughout the system makes it a local quantity, as opposed to frequency,
which is a global quantity, because it is identical in the whole system. Voltage problems
must always be solved at or in the vicinity of the node at which the problem occurs, as
opposed to frequency problems that can be solved everywhere, provided that no branch
overloadings occur.
Voltage control is only possible at those nodes in the system at which a device is
installed which is capable of changing the phase angle between the terminal voltage and the
injected current. The most frequently occurring example of such a device is the
synchronous generator used in large scale centralized power plants. The angle between
terminal voltage and injected current is dependent on the current in the rotor winding.
Main Points
The main points addressed in this chapter are:
• General description of the voltage control problem, including definitions of active and
reactive power and power factor
• Brief description of the most frequently applied wind turbine types, with a focus on their
voltage control capabilities
• Quantitative analysis of the voltage control capabilities of each of the wind turbine
types, both steady state and dynamically
• Drawback of voltage control with wind turbines: an increased converter rating in both
variable speed wind turbine types